Robinson Crusoe Island has a mountainous and undulating terrain, formed by ancient lava flows which have built up from numerous volcanic episodes. The highest point on the island is 915 m (3,002 ft) above sea level at El Yunque. Intense erosion has resulted in the formation of steep valleys and ridges. A narrow peninsula is formed in the southwestern part of the island called Cordón Escarpado. The island of Santa Clara is located just off the southwest coast.
Robinson Crusoe Island lies to the west of the boundary between the Nazca Plate and the South American Plate, and rose from the ocean 3.8 – 4.2 million years ago. A volcanic eruption on the island was reported in 1743 from El Yunque, but this event is uncertain. On 20 February 1835, a day-long eruption began from a submarine vent 1.6 kilometres (1.0 mi) north of Punta Bacalao. The event was quite minor—only a Volcanic Explosivity Index 1 eruption—but it produced explosions and flames that lit up the island, along with tsunamis.